Liquid carbon dioxide refrigeration control system



V. GUIFFRE Filed Aug. 16, 1966 LIQUID CARBON DIOXIDE REFRIGERATIONCONTROL SYSTEM Sept. 10, 1968 United States Patent O 3,400,550 LIQUIDCARBON DIOXIDE REFRIGERATION CONTROL SYSTEM Vincent Guitfre, Ramsgate,New South Wales, Australia, assignor to The Colonial Sugar ReniugCompany Limited, Sydney, New South Wales, Australia, a company of NewSouth Wales, Australia Filed Aug. 16, 1966, Ser. No. 572,755 Claimspriority, application Australia, Aug. 25, 1965, 63,273/ 65 6 Claims.(Cl. 62-64) ABSTRACT F THE DISCLGSURE Method of and apparatus formaintaining pre-cooled goods (for example, meat) during storage in achamber at a temperature substantially the same as the average initialpost-storage temperature of the goods. The method comprises tirstsensing the average air temperature near the iioor of the storagechamber `at a predetermined time after loading, then sensingcontinuously the average air temperature near the roof of the chamber,and refrigerating the chamber responsively to reduce the differencebctween the two sensed temperatures.

The method and apparatus according to the invention have been devised tomeet the minimum refrigeration requirements for the successful storageof pre-cooled goods during their bulk transport. In general, theinvention is described hereinafter with respect to the storage of goodsin refrigerator railway carriages, but it will be understood that thisparticular application of the invention is described by way ofillustration only.

After loading into a refrigerator railway carriage, the averagetemperature of a cargo of goods is not usually known, nor can it bereadily measured. Factors which have a bearing on this averagepost-loading temperature 'are numerous and include: storage time andstacking methods used in a previous cool-room, refrigeration capacity ofthat cool-room, and conditions prevailing during transfer from cool-roomto railway carriage. Thus, in the case of pre-cooled meat, chilling in acool-room may have lowered the surface temperature of the meat to about36 F., but the average temperature after loading into a rerigeratiorrailway carriage may be as high as about 45 F.

According to refrigeration procedures currently practiced, thetemperature control mechanism in the railway carriage is set to providean arbitrarily selected temperature which may bear little relation tothe average temperature of the cargo immediately after loading. If thearbitrarily selected temperature is much lower than this averagetemperature, a considerable and unnecessary strain is imposed on therefrigerating mechanism.

This known method is also subject to an additional disadvantage in thatit involves the possibility of error on the part of the operator who isrequired to set the temperature control mechanism. In the case ofpre-cooled meat, for example, the arbitrarily selected temperature maybe 38 F., but the operator may inadvertently set the temperature at,say, 36 F. When refrigeration is aciheved directly by spraying liquidcarbon dioxide or liquid nitrogen on the cargo from the roof of thecarriage, a total of about 150 lb. of refrigerant may be expended inreducing the temperature of an average load of chilled meat by lFahrenheit degree. It can be seen therefore that errors in the selectionof an appropriate storage temperature can be magnified by errors on thepart of the operator, thus resulting in an appreciable overall wastageof refrigerant.

It is an object of the present invention to overcome these disadvantagesby providing a method of refrigeration designed to maintainautomatically during storage substantially the average initialpost-storage temperature of the pre-cooled goods.

The invention is based on the following premises:

(i) Pre-cooled goods are delivered to a storage chamber-say, a railwayrefrigerator carriage--at an average temperature lying in a temperaturerange considered to be optimum for the goods in question;

(ii) Depending on whether the air temperature in the carriage isinitially greater than or less than the temperature of the cargo, theaverage temperature detected near the door of the carriage will decreaseor increase gradually after loading until it reaches a terminal valueapproximating the average initial post-storage temperature of the goods;

(iii) The period of time needed for this terminal condition to bereached can be predicted reliably from reference experiments in whichthe rate of temperature change is recorded for loading with goods of thegiven kind;

(iv) After the terminal condition has been reached, the minimumrefrigeration requirements for storage during transport are satisfied ifthe carriage is then refrigerated to such an extent that the averagetemperature detected near the roof of the carriage is lowered towardsthe detected terminal value of the average temperature near the iioor.

Broadly, the invention provides a method of maintaining pre-cooled goodsduring storage at substantially the average initial post-storagetemperature of said goods, said method comprising: loading a storagechamber with said goods; sealing the chamber; sensing continuously afirst average air temperature towards the tioor of the chamber during apredetermined iirst period of time; sensing continuously a secondaverage air temperature towarls the roof of the chamber during asubsequent second period of time; comparing continuously during saidsecond period of time said second sensed average air temperature withthe terminal value of said first sensed average air temperature, andrefrigerating the chamber responsively to reduce the differencetherebetween.

The invention also provides apparatus for performing the defined method,said apparatus being adapted for installation in association with astorage chamber and comprising: a refri-gerating control mechanism;first means for sensing continuously a first average air temperaturetowards the iioor of the chamber during a predetermined first period oftime and for supplying simultaneously said refrigerating controlmechanism with a rst temperature signal continuously indicative of saidiirst sensed average air temperature; second means for sensingcontinuously a second average air temperature towards the roof of thechamber during a subsequent second period of time and for supplyingsimultaneously said refrigerating control mechanism with a secondtemperature signal continuously indicative of said second sensed averageair temperature; a timing device for actuating said first temperaturesensing means during said predetermined first period of time and forthen actuating said second temperature sensing means; comparator meansincluded in said refrigerating control mechanism for comparingcontinuously said second temperature signal with the terminal value ofsaid first temperature signal; refrigerating means associated with saidcomparator means for refrigerating the chamber responsively to reducethe difference between said second temperature signal and the terminalvalue of said first temperature signal.

According to a preferred embodiment of the invention, the iirst andsecond temperature sensing means are adapted for pneumatic actuation bythe timing device and the timing device is itself pneumaticallyoperated.

In this embodiment, the timing device comprises: a gas storage reservoirlinked by first pipe means to a source of compressed gas; said firstpipe means including a diaphragm-operated valve for controlling gas liowfrom said source to said reservoir; means for effecting the closure ofsaid diaphragm-operated valve when a pre-selected high. pressure hasbeen attained in said reservoir, said closure means comprising secondpipe means for enabling closureeffecting communication between saidreservoir and the diaphragm of said diaphragm-operated valve, saidsecond pipe means including a non-return valve openable when saidpre-selected high pressure has been attained in said reservoir; ventingmeans for effecting the re-opening as required of saiddiaphragm-operated valve, said venting means being provided in thesecond pipe means between said non-return valve and the diaphragm ofsaid diaphragm-operated valve and being adapted on actuation to reducethe pressure on said diaphragm; third pipe means linking said reservoircommunicatively with the actuating mechanisms of said first and secondtemperature sensing means; bleeding means associated with said reservoirfor enabling gas to escape therefrom at a controlled; rate whereby toreduce the pressure therein from said preselected high value towards andbelow a preselected llow value.

Conveniently, the actuating mechanism of one of the temperature sensingmeans (say, the first temperature sensing means) is adapted to operatewhen the pressure in the reservoir is in excess of this preselected lowvalue, and the actuating mechanism of the other (second) temperaturesensing means is adapted to operate when the pressure in the reservoiris less than the preselected low value.

The first and second temperature sensing means are hereinafter referredto as floor and roof temperature sensing means, respectively.

The functioning of the timing device will be explained more fully in thedescription relating to the annexed drawing. It can be seen, however,that the passage of gas from the source to the reservoir results in anincreasing pressure therein, firstly until a value is reached at whichthe fioor temperature sensing means is actuated, and secondly until amaximum value is reached at which the inflow of gas to the reservoir isterminated. This maximum pressure exceeds the low pressure at which theroof temperature sensing means is adapted to be actuated, butprogressive loss of gas to atmosphere via the bleeding mechanism ensuresthat the pressure in the reservoir returns slowly to a value below whichactuation occurs. The bleeding mechanism can be adjusted if necessary todelay actuation of the roof temperature sensing means to a variableextent, the period of delay being predetermined to suit the requirementsof the cargo in question.

Provision of the described venting means in the timing device enables itto be returned to its initial condition when the required storage periodis at an end.

The required degree of refrigeration can be effected by making use ofany suitable refrigeration system, by way of illustration: compression,absorption, or expendable liquefied gas refrigeration systems.

An expendable liquefied gas refrigeration system is employed in theembodiment of the invention now to be described. According to thissystem, refrigeration is effected by (a) vapourizing a suitableliquefied gas in a heat exchanger within the storage chamber (indirectcooling) prior to discharging to atmosphere, or by (b) vapourizing asuitable liquefied gas in a heat exchanger as in (a), and then sprayingthe vapourized gas at low temperature into the storage chamber (directcooling).

It will be appreciated that technique (b) is not favoured when the cargocomprises goods (e.g., chilled meat) which are affected detrimentally byan atmosphere depleted in oxygen; however, in technique (a) thedischarge site can be arranged outside the storage chamber, in whichcase the atmosphere within the chamber does not become depleted inoxygen. i

Liquid nitrogen and liquid carbon dioxide both exemplify suitable gasesfor use in an expendable liquefied gas refrigeration system. Liquidnitrogen has generally been favoured for industrial refrigerationpurposes, however, in the embodiment of the invention now to bedescribed, liquid carbon dioxide is both the refrigerant and the sourceof gas for operating the (pneumatic) timing device.

The drawing shows schematically a system comprising apparatus foreffecting minimum refrigeration according to the invention.

This system has been designed for installation in association with arailway refrigerator carriage (not shown), and consists of a combinationof parts including: a vessel 1 for storing liquid carbon dioxiderefrigerant; means consisting of a temperature sensing element 29 forsensing the average air temperature towards the fioor of the chamber;means consisting of a temperature sensing element 36 for sensing theaverage air temperature towards the roof of the chamber; a refrigeratingcontrol mechanism comprising a pneumatic temperature transmitter 30 anda pneumatic temperature controller 35; a pneumatic timing devicecomprising a reservoir 20 for gaseous carbon dioxide and a constantbleed mechanism 21; refrigerating eans selected as hereinafter explainedfrom the group consisting of heat exchanger cooling means 38 and directspray cooling means 39; selector valve 37 for selecting the desiredrefrigerating means; and master control valve 16 for isolating asrequired the entire control system from the vessel storing liquid carbondioxide.

Summarised operation.

The refrigerant supply vessel having been filled with liquid carbondioxide and a cargo having been loaded into the storage chamber, theoperator switches on the refrigeration control system by adjusting themaster control valve 16, and selects the desired refrigerating means byadjusting selector valve 37. No further adjustments are necessary.

The pneumatic timing device now automatically measures a predeterminedperiod of time during which the pneumatic temperature controller 35 issupplied via pneumatic temperature transmitter 30 with a pressure signalfrom floor temperature sensing element 29.

Subsequent to this measured period, the pneumatic temperature controlleris supplied instead with a pressure signal from roof temperature sensingelement 36. A comparison is then continuously made by the temperaturecontroller between the terminal value of the sensed temperature near thefloor with the prevailing value of the sensed temperature near the roof,and the refrigerating means is brought into action as required to reducethe difference between the two temperatures to zero. Other partsincluded in the system are discussed below in relation to variousoperational aspects.

Refrgerant supply Liquid carbon dioxide, at a pressure of about p.s.i.g.and a temperature of about 51 F., is supplied to vacuum insulated vessel1 from liquid filling line 2 coupled to the inner pipe of a twinconcentric feeding hose 3. The liquid is passedfthrough strainer 4 andthe supply is controlled by manually operated valve 5.

Gas displaced from the vessel` is returned by gas vline 6 to themanually operated valve 5 and thence to the annular space between theinner and outer'pipes ofthe feeding hose. When the filling operation iscompleted, liquid carbon dioxide begins to return by the gas lineto thefeeding hose, and the manually operated valve 5 is then shut.Confirmation that this stage has been reached is obtained readily byopening try-cock 7 and testing for the presence therein of liquid carbondioxide.

During those periods when refrigeration is not required in the storagechamber, the refrigerant supply system is isolated from the rest of theapparatus and dangerous pressures are liable to be generated therein. Tocounter this possibility, the gas-containing section of vessell 1 isattached via heating coil 8 to a bleed valve 9 designed to open atpressures exceeding, say 105 p.s.i.g.

Communication between the refrigerant supply vessel and therefrigerating means occurs via ball valve 10, strainer 11 and non-returnvalve 12 in liquid supply line 13 Communication between the refrigerantsupply vessel and the refrigerating control system occurs by way ofliquid filling line 2 through heating coil 14, strainer 15 and mastercontrol valve 16.

Manual adjustments The refrigerant supply vessel having been filled withliquid carbon dioxide and a cargo having been loaded into the storagechamber, the operator switches on the refrigerating control system byadjusting the master control valve 16, and selects the desiredrefrigerating means by adjusting selector valve 37. The system is nowunder automatic control and no further adjustments are necessary.

When communication is effected between the refrigerant supply vessel andthe refrigerating control system, a small quantity of liquid carbondioxide is vapourized in the heating coil 14. The gas is thentransmitted (at about 100 p.s.i.g.) by pipe line 17 towards therefrigerating control mechanism, and by pipe line 18 towards thepneumatic timing device.

Pneumatic timing A diaphragm-operated valve 19, included in pipe line18, is open at the start of the operation. Gas from pipe line 18 cantherefore flow through this valve into reservoir 20 and its associatedpiping.

The reservoir communicates with a gas bleeding mechanism 21 and with thediaphragms of diaphragmoperated Valves 22 and 23. A pipe line 24, 25,including non-return valve 26, is provided to enable delayedcommunication between the reservoir and the diaphragm ofdiaphragm-operated valve 19.

When the pressure in the reservoir builds up to 3 p.s.i.g., thearrangement is such that diaphragm-operated valve 22 is closedand`disphragm-operated valve 23 is opened. This actuation marks thebeginning of a timed interval (referred to above as the predeterminedfirst period of time).

Non-return valve 26 is adapted to be opened when the pressure in thereservoir builds up to 60 p.s.i.g., and diaphragm-operated valve 19 isadapted to be closed when pressure on its diaphragm exceeds 20 p.s.i.g.Accordingly, a stage is soon reached at which gas is admitted from thenon-return valve to the diaphragm of diaphragm-operated valve 19.Closure of this valve is thereby effected and further supply of gas tothe reservoir is then prevented. Pressure in the reservoir now returnsgradually to normal by virtue of continuous loss of gas from bleedingmechanism 21. In the preferred embodiment shown in the drawing, gaspassing through non-return valve 26 is admitted additionally to thediaphragm of another diaphragm-operated valve 27. The latter valve isincluded in a pipe line 28 interconnecting pipe line 18 and section 25of pipe line 24, 25, and is adapted to be opened when pressure on itsdiaphragm exceeds 3 p.s.i.g. Accordingly, when gas at a pressure ofabout 60 p.s.i.g. passes from non-return valve 26 to the diaphragm ofdiaphragmoperated valve 27, the latter valve is opened and communicationis effected between gas line 18 and the diaphragm of diaphragm-operatedvalve 19. Since the lgas in gas line 18 is at about 100 p.s.i.g., it canbe ensured by this arrangement that diaphragm-operated valve 19 willstay closed after the pressure in reservoir 20 has returned to normal.(If this refinement is omitted, the

possible development of minute leaks in the system could lead to there-opening of the diaphragm-operated valve 19 as the pressure in thereservoir returns to normal.) As will be explained hereinafter,diaphragm-operated valve 27 and pipe line 28 also constitute ventingmeans by which the pressure acting on the diaphragm of diaphragmoperatedvalve 19 can subsequently be released.

Diaphragm-operated valves 22 and 23 are again actuated when the pressurein reservoir 20 is reduced to 3 p.s.i. g., valve 22 now opening andvalve 23 now closing. The interval between the two actuations of thesevalves is a constant determined essentially by the dimensions of thereservoir 20, the maximum pressure attained therein, and the orificecharacteristics of bleeding mechanism 21.

Temperature sensing and control A temperature sensing element 29 ismounted near the floor of the storage chamber. This element consists ofa long capillary-type device adapted to supply pneumatic temperaturetransmitter 30 with a pressure signal whose magnitude depends on theaverage air temperature in its vicinity.

Diaphragm-operated Valve 22 is closed at the beginning of the timedinterval, however gas from pipe line 17 flows to pneumatic temperaturetransmitter 30 through non-return valve 31 and pressure regulator 32.The regulator serves to reduce the pressure of this gas before enteringthe transmitter from about p.s.i.g. to about 20 p.s.i.g.

Diaphragm-opearted valve 23 is open at the beginning of the timedinterval, and the output pressure signal from temperature transmitter 30therefore passes from pipe line 33 to temperature compensating device 34and pneumatic temperature controller 35. (The temperature compensatingdevice comprises an insulated reservoir, its function being tocompensate for externally induced irregularities in the pressure of gasflowing through pipe line 33.) This output pressure signal iscontinuously indicative of the sensed average air temperature near thefloor of the chamber and gradually approaches a terminal value as thisair temperature approaches the average initial post-storage temperatureof the cargo.

As mentioned, diaphragm-operated valves 22 and 23 are 4opened and closedrespectively at the end of the timed interval. At this stage,temperature transmitter 30 ceases to supply temperature controller 35with a pressure signal, but the latter is now -brought into action bygas flowing through valve 22.

A temperature-sensing element 36 is mounted near the roof of the storagechamber and is of the same type as previously described element 29. Thiselement is adapted to supply temperature controller 35 with a pressuresignal continuously indicative of the average air temperature near theroof.

Comparator means (not shown) are provided in the temperature controllerfor comparing the terminal value of the sensed temperature near thefloor with the prevailing value of the sense-d temperature near theroof, and-if the latter exceeds the former-an output pressure signal ofabout 20 p.s.i.g. is passed from the temperature controller to selectorvalve 37.

Depending on the setting of this valve, the output signal serves toactuate (a) heat exchanger refrigeration means 38, or (b) heat exchangermeans 38 and direct spray cooling means 39. When the difference betweenthe two temperatures is reduced to zero, the output signal from thetemperature controller is cut off and refrigeration ceases. When thesensed temperature near the roof again exceeds the terminal value of thesensed temperature near the oor, an output signal is again transmittedfrom the temperature controller and refrigeration is recommenced.

Refrigeration Heat exchanger refrigeration means 38 communicatescontinuously by pipe line 13 with the refrigerant supply vessel 1.

Means are provided for effecting communication as desired between thedownstream side of the heat exchanger and V(i) the external atmosphereor (ii) the atmosphere within the storage chamber. In the former case,the consequence of communication is refrigeration by indirect cooling(heat exchanger cooling) and discharge of vapourized carbon dioxide tothe external atmosphere; in the latter case, the consequence ofcommunication is refrigeration by indirect cooling (heat exchangercooling) and also refrigeration by direct cooling (spray cooling), therenow being discharge of vapourized carbon dioxide into the chamber.

Discharge to the external atmosphere is effected via pipe lines 40, 41,and is controlled by diaphragm-operated valve 42. The diaphragm of thelatter is linked to selector valve 37 by pipe line 43. As shown in thedrawing, the selector valve is set in such a way that a pressure signalfrom the temperature controller 35 can be transmitted by this pipe lineto the diaphragm, thereby opening valve 42 and enabling discharge ofvapourized carbon dioxide to the external atmosphere. The sustainedpassage of vapourizing carbon dioxide through the heat exchanger effectsa gradual reduction in the temperature of the storage chamber.

During periods of refrigeration, excessive pressures are liable to begenerated from time to time in the heat exchanger refrigeration system,and for this reason, pipe line 4t) is attached via heating coil 44 to arelief valve 45 designed to open at pressures exceeding, say, 120p.s.i.g.

Discharge into the storage chamber is effected via pipe lines 40, 46, 47and spray cooling means 39, and is controlled by diaphragm-operatedvalve 48. The diaphragm of the latter is linked to selector valve 37 bypipe line 49, and-when the selector valve is set in the alternativeposition to that shown in the drawing-a pressure signal can betransmitted to this diaphragm to open valve 48.

A gas reservoir 50 communicates continuously with pipe line 47 on thedownstream side of valve 48. When the valve is opened therefore,vapourized carbon dioxide is released both into the pipe line and intothe associated reservoir, and is discharged through the nozzles of thespray cooling means into the storage chamber.

If heat exchanger 38 is comparatively small, and if diaphragm-operatedvalve 48 is maintained in the open position for a prolonged period, theproportion of vapourized carbon dioxide lilling the heat exchangergradually diminishes and its place is taken by liquid carbon dioxide. Asthis process continues, carbon dioxide is eventually discharged as aliquid through the nozzles of the spray-cooling means; the reservoir,however, remains substantially charged with vapourized carbon dioxide.

Car-bon dioxide has a triple point pressure of about 60 p.s.i.g. atabout 70 F. For this reason, liquid carbon dioxide is susceptible to DryIce formation at the comparatively low pressure contemplated by thepresent embodiment.

When diaphragm-operated valve 48 is closed at the end of a dischargephase, the pressure in pipe line 47 begins to fall and there is anincreasing possibilty of Dry Ice precipitation within the spray coolingsystem. Extensive nozzle blockages would therefore be expected to occurat this stage.

It has been ascertained, however, that the reservoir can provide asource of auxiliary gas at a pressure sufficient to flush residualliquid carbon dioxide and particles of precipitated Dry Ice through thedischarge nozzles. As a result, blockages do not occur.

It will be appreciated that this reservoir has a critical minimum volumewhich must be exceeded to enable satisfatcory operation under a givenset of conditions. The critical volume can readily be determined bytrial and error.

In the illustrated embodiment, the spray-cooling means consists of alooped arrangement including appropriate nozzles throughout its length,and carbon dioxide is distributed from a centrally located pipe toopposed segments of the loop. By adopting this arrangement there is amuch diminished possibility of refrigeration being disrupted in theentire spray cooling system by the occurrence of random nozzle blockagesin a part of the system.

T @rmi/ration When the storage period is at an end, the master controlvalve 16 is switched off and gas pressure in pipe lines 17 and 1S isreduced to normal by venting through discharge means 51. As the pressuredrops from about p.s.i.g. to about 20 p.s.i.g., diaphragm-operated valve19 is opened; and at 3 p.s.i.g diaphragm-operated valve 27 closes.

The entire system is thus returned to its original condition.

As an additional refinement, it has been found advantageous to associatepipe line 25 with a quick-release valve 52.

By arranging for this valve to open at some pressure between 20 p.s.i.g.and 3 p.s.i.g., say at l5 p.s.i.g., it can be ensured that the pressureacting on the diaphragm of valve 27 is rapidly released .and that thisvalve is therefore rapidly closed. Consequentially, there is nolikelihood that premature re-setting of the master control valve cancause the unwanted closure of valve 19.

I claim:

1. A method Iof maintaining pre-cooled goods during storage atsubstantially the average initial post-storage temperature of saidgoods, said method comprising: loading a storage chamber with saidgoods; sealing the chamber; sensing continuously a first average airtemperature towards the lioor of the chamber during a predeterminedfirst period of time; sensing continuously a second average airtemperature towards the roof of the chamber during a subsequent sec-ondperiod of time; comparing continuously during said second period of timesaid second sensed average air temperature with the terminal value ofsaid first sensed [average air temperature and refrigerating the chamberresponsively to reduce the difference therebetween.

2. Apparatus for maintaining pre-cooled goods during storage atsubstantially the average initial post-storage temperature of saidgoods, said apparatus being adapted for installation in association witha storage chamber and comprising: a refrigerating control mechanism;iirst means for sensing continuously la first average air temperaturetowards the oor of the chamber during a predetermined first period oftime and for supplying simultaneously `said refrigerating controlymechanism with a first ternperature signal continuously indicative ofsaid first sensed average air temperature; second means for sensingcontinuously a second average air temperature towards the roof of thechamber during a subsequent second period of time and for supplyingsimultaneously said refrigerating control mechanism with a secondtemperature signal continuously indicative of said second sensed averageair temperature; a timing device for actuating said irst temperaturesensing means during said predetermined first period of time and forthen actuating said second temperat-ure sensing means; comparator meansincluded in said refrigerating control mechanism for comparingcontinuously said second temperature signal with terminal value of saidfirst temperature signal; refrigerating Imeans connected to saidcomparator means for refrigerating the chamber responsively to reducethe difference between said second temperature signal and the terminalvalue of said first temperature signal.

3. Apparatus according to claim 2, wherein said first and secondtemperature sensing means are pneumatically actuated by said timingdevice and wherein said timing device is pneumatically operated.

4. Apparatus according to claim 3, wherein said timing device comprises:.a gas storage reservior linked by rst pipe means to a source ofcompressed gas; said rst pipe means including a diaphragm-operated valvefor controlling gas ow from said source to said reservoir; means foreffecting the closure of said diaphragm-operated valve when :apreselected high pressure has been attained in said reservoir, saidclosure means comprising second pipe means for enablingclosure-effecting communication between said reservoir and the diaphragmof said diaphragmoperated valve, said second pipe means including anonreturn Valve openable when said preselected high pressure has beenattained in said reservoir; venting means for effecting the re-openingas required of said diaphragm-operated valve, said venting means beingprovided in the second pipe means between said non-return valve and thediaphragm of said diaphragm-operated valve and being adapted onactuation t-o reduce the pressure on said diaphragm; third pipe meanslinking said reservoir communicatively with the actuating mechanisms ofsaid rst and second temperature sensing means; bleeding means associatedwih said reservoir for enabling gas to escape therefrom at a controlledrate whereby to reduce the pressure therein from said preselected highvalue towards and below a preseelcted low value.

5. Apparatus according to claim 4, wherein the actuating mechanism ofone said temperature sensing means is adapted to operate when thepressure in the reservoir is in excess of said preselected low value,and the actuating mechanism of the other said temperature sensing meansis adapted to operate when the pressure in said reservoir is less thansaid preselected low value.

`6. Apparatus according to claim 5, wherein said refrigerating meanscomprises direct spray cooling means, the refrigerant therefor beingprovided by liquid carbon dioxide.

References Cited UNITED STATES PATENTS 2,531,136 11/1950 Kurtz 62-209 X3,281,075 10/ 1966 Smyers 236-80 X 3,287,925 11/1966 Kane et al. 62-64 XROBERT A. OLEARY, Primary Examiner. W. E. WAYNER, Assistant Examiner.

